1. Field of the Invention
The present invention concerns printing flat objects such as compact discs or computer diskettes in one or more colors and is more particularly concerned with the situation in which, to obtain a particularly fine quality of printing, the printing machine utilizes at least one offset printing head.
2. Description of the Prior Art
An offset printing head includes at least one inking roller and at least one blanket cylinder mounted to rotate parallel to each other on a common carriage that is mobile in translation on a frame. As it moves to and fro, the carriage sweeps across a plate support adapted to receive a plate and an object support adapted to support an object to be printed.
At present offset printing heads are in practise entirely mechanical and in particular rotation of the inking roller and the blanket cylinder are mechanically induced by the movement of the carriage, either because they are mounted as idler rollers on the carriage and are rotated purely because of their contact with the plate and, in the case of the blanket cylinder, with the object to be printed, or because they are positively rotated by a drive device of some kind, for example a rack and pinion device, fixed to the carriage.
The execution of these various movements is therefore highly rigid, and the same applies to the corresponding adjustments, to the detriment of flexibility of operation and use of the system.
To increase the thickness of the film of ink deposited onto the plate and thereby to improve the inking of the plate, two spaced parallel inking rollers are usually employed.
This increases the length of the carriage, however, to the detriment of the amount of floor space taken up by the printing machine and of the duration of its operating cycle, and therefore its productivity.
At the cost of some complexity, it is at present normally necessary to provide for relative vertical movement between the blanket cylinder and the object support in offset printing heads so that, after the object it carries is printed, the object support can be moved back without risk of interference with the blanket cylinder and therefore without risk of soiling the printed object, and so that a new object support carrying a new object to be printed can be offered up at the appropriate time and under the same conditions.
A general object of the present invention is an arrangement that avoids these drawbacks and has other advantages.